通信外文翻译外文文献英文文献及译文

1、通信外文翻译外文文献英文文献及译文Communication SystemA generalized communication system has the following components:(a) Information Source. This produces a message which may be written or spoken words, or some form of data.(b) Transmitter. The transmitter converts the message into a signal, the form of which is su

2、itable for transmission over the communication channel.(c) Communication Channel. The communication channel is the medium used transmit the signal, from the transmitter to the receiver. The channel may be a radio link or a direct wire connection.(d) Receiver. The receiver can be thought of as the in

3、verse of the transmitter. Itchanges the received signal back into a message and passes themessage on to its destination which may be a loudspeaker, teleprinter orcomputer data bank.An unfortunate characteristic of all communication channels is that noise is added to the signal. This unwanted noise m

4、ay cause distorions of sound in a telephone, or errors in a telegraph message or data.Frequency Diversion MultiplexingFrequency Diversion Multiplexing(FDM) is a one of analogtechnologies. A speech signal is 03 kHz, single sideband amplitude (SSB) modulation can be used to transfer speech signal to n

5、ew frequency bands,four similar signals, for example, moved by SSB modulation to share the band from 5 to 20 kHz. The gaps between channels are known as guard spaces and these allow for errors in frequency, inadequate filtering, etc in the engineered system.Once this new baseband signal, a "gro

6、up" of 4 chEmnels, has been foimed it ismoved around the Lrunk network as a single unit. A hierarchy can be set up withseveral channels fonning a "group". several groups a "supergroup" and several"supergraup" eicher a "nmsrergroup" or "hypergroup&quo

7、t;.Groups or supergroups are moved around as single units by the communicationsequipment and it is not necessary for the radios to know how many channels are involved. A radio can handle a supergroup provided sufficient bandwidth is available. The size of the groups is a compromise as treating each

8、channel individually involves far moreequipment because separate filters, modulators and oscillators are required for every channel rather than for each group. However the failure of one module will lose all of the channels associated with agroup.Time Diversion MultiplexingIt is possible, with pulse

9、 modulation systems, to use the between samples to transmit signals from other circuits. The technique is known as time diversion multiplexing (TDM). To do this, it is necessary to employ synchronized switches at eachend of the communication links to enable samples to be transmitted in turn, from ea

10、ch of several circuits. Thus several subscribers appear to use the link simultaneously. Although each user only has periodic short time slots, the original analog signalsbetween samples can be reconstituted at the receiver.Pulse Code ModulationIn analog modulation, the signal was used to modulate th

11、e amplitude or frequency of a carrier, directly. However, in digital modulation a stream of pulse, representing the original, is created. This stream isthen used to modulate a carrier or alternatively is transmitted directly over a cable. Pulse Code Modulation (PCM) is one of the two techniques comm

12、only used.All pulse systems depend on the analog waveform being sampled at regular intervals. The signal created by sampling our analog speech input is known as pulse amplitude modulation. It is not very useful in practice but is used as an intermediate stage towards forming a PCM signal. It will be

13、 seen later that most of the advantages of digital modulation come from the transmitted pulses having two levels only, this being known as a binary system. In PCM the height of each sample is converted into a binary number. There are three step in the process of PCM: sampling, quantizing and coding.

14、Optical Fiber CommunicationsCommunication may be broadly defined as the transfer of information from one point to another. When the information is to be conveyed over any distance acommunication system is usually required. Within a communication system the information transfer is frequently achieved

15、 by superimposing or modulating the information on to an electromagnetic wave which acts as a carrier for the informationsignal. This modulated carrier is then transmitted to the required destination where it is received and the original information signal is obtained by demodulation. Sophisticated

16、techniques have been developed for this process by using electromagnetic carrier wavesoperating at radio frequencies as well as microwave and millimeterwave frequencies. However , 拔 communication?may also be achieved by using an electromagneticcarrier which is selected from the optical range of freq

17、uencies.In this case the information source provides an electrical signal toa transmitter comprising an electrical stage which drives an optical source to give modulation of the light-wave carrier. The optical source which provides the electrical-optical conversionmay be either a semiconductor laser

18、 or light emitting diode (LED).The transmission medium consists of an optical fiber cable and the receiver consists of an optical detector which drives a further electrical stage and hence provides demodulation optical carrier. Photodiodes (P-N, P-I-N or avalanche) and , in some instances,phototrans

19、istor and photoconductors are utilized for the detection of the optical signal and the electrical-optical conversion. Thus there is a requirement for electrical interfacing at either end of the optical link and at present the signal processing is usually performed electrically.The optical carrier ma

20、y be modulated by using either an analog or digital information signal. Analog modulation involves the variation of the light emitted from the optical source in a continuous manner. With digital modulation, however, discrete changes in the light intensity are obtained (i.e. on-off pulses). Although

21、often simpler toimplement, analog modulation with an optical fiber communication system is lessefficient, requiring a far higher signal to noise ratio at the receiver than digital modulation. Also, the linearity needed for analog modulation is not always provided by semiconductor optical source, esp

22、ecially at high modulation frequencies. For thesereasons , analog optical fiber communications link are generally limited to shorter distances and lower bandwidths than digital links.Initially, the input digital signal from the information source is suitably encoded for optical transmission. The las

23、er drive circuit directly modulates the intensity of the semiconductor laser with the encoded digital signal. Hence a digital optical signal islaunched into the optical fiber cable. The avalanche photodiodedetector (APD) is followed by a fronted-end amplifier and equalizer or filter to provide gain

24、as well as linear signal processing and noise bandwidth reduction. Finally, the signal obtained isdecoded to give the original digital information.Mobile CommunicationCordless Telephone SystemsCordless telephone system are full duplex communication systems that use radio to connect a portable handse

25、t to a dedicated base station which is then connected to adedicated telephone line with a specific telephone number on the public switched telephone network (PSTN) .In first generation cordless telephone systems5(manufactured in the 1980s), the portable unit communications only to the dedicatedbase

26、unit and only over distances of a few tens of meters.Early cordless telephones operate solely as extension telephones to a transceiver connected to a subscriber line on the PSTN and are primarily for in-home use.Second generation cordless telephones have recently been introduced which allowsubscribe

27、rs to use their handsets at many outdoor locations within urban centers such as London or Hong Kong. Modern cordless telephones are sometimes combined with paging receivers so that a subscriber may first be paged and then respond to the pageusing the cordless telephone. Cordless telephone systems pr

28、ovide the user with limited range and mobility, as it is usually not possible to maintain a call if the user travels outside the range of the base station. Typical second generation base stations provide coverage ranges up to a few hundred meters.Cellular Telephone SystemA cellular telephone system

29、provides a wireless connection to the PSTN for any user location within the radio range of the system.Cellular systems accommodate alarge number of users over a large geographic area, within a limited frequency spectrum. Cellular radio systems provide high quality service that is often comparable to

30、 that of the landline telephone systems. High capacity is achieved by limiting the coverage of each base station transmitter to a small geographic area called a cell so that the same radio channels may be reused by another base station located some distance away. A sophisticated switching technique

31、called a handoff enables a call to proceeduninterrupted when the user moves from one cell to another.A basic cellular system consists of mobile station, base stations and a mobile switching center (MSC). The Mobile Switching Center is sometimes called a mobiletelephone switching office (MTSO), since

32、 it is responsible for connecting all mobiles to the PSTN in a cellular system. Each mobilecommunicates via radio with one of the base stations and may be handed- off to any number of base stations throughout the durationof a call. The mobile station contains a transceiver, an antenna, and control c

33、ircuitry , and may be mounted in a vehicle or used as a portable hand-held unit. Thebase stations consists of several transmitters and receivers which simultaneously handlefull duplex communications and generally have towers which support several transmitting and receiving antennas. The base station

34、 serves as a bridge between all mobile users in the cell and connects the simultaneous mobile calls via telephone linesor microwave links to the MSC. The MSC coordinates the activities of all the base stations and connects the entire cellular system to the PSTN. A typical MSC handles 100000 cellular

35、 subscribers and 5000 simultaneous conversations at a time, andaccommodates all billing and system maintenance functions, as well.In large cities, several MSCs are used by a single carrier.Broadband CommunicationAs can be inferred from the examples of video phone and HDTV, the evolution offuture com

36、munications will be via broadband communication centered around video signals. The associated services make up a diverse set of high-speed and broadband,videoservices ranging from video services such as video phone conferencing , videosurveillance, cable television (CATV) distribution, and HDTV dist

37、ribution to the high-speed data services such as high-resolution image transmission, high-speed datatransmission, and color facsimile. The means of standardizing these various broadbandcommunication services so that they can be provided in an integrated manner is no other than the broadband integrat

38、ed services digital network (B-ISDN). Simple put, therefore,the future communications network can be said to be a broadband telecommunicationsystem based on the B-ISDN.For realization of the B-ISDN, the role of several broadband communicationtechnologies is crucial. Fortunately, the remarkable advan

39、ces in the field of electronics and fiber optics have led to the maturation of broadband communication technologies.As the B-ISDN becomes possible on the optical communication foundation, the relevant manufacturing technologies for light-source and passive devices and for optical fiberhave advanced

40、to considerable levels. Advances in high-speed device and integratedcircuit technologies for broadband signal processing are also worthy of close attention. There has also been notable progress in software, signal processing, and video equipment technologies. Hence, from the technological standpoint

41、, the B-ISDN hasfinally reached a realizable state.On the other, standardization activities associated with broadband communication have been progressing. The Synchronous Optical Network (SONET) standardization centered around the T1 committee eventually bore fmit in the form of the Synchronous Digi

42、tal Hierarchy (SDH) standards of the International Consultative Committee in Telegraphy and Telephony (CCITT), paving the way for synchronous digital transmission based on optical communication. The standardization activities of the 5integrated services digital network (ISDN), which commenced in ear

43、ly 1980s with the objective of integrating narrowband services, expanded in scope with the inclusion of broadband services, leading to the standardization of the B-ISDN in late1980 担 and establishing the concept of asynchronous transfer mode (ATM)communication in process. In addition, standardizatio

44、n of various video signals is becoming finalized through the cooperation among such organizations as CCITT, the International Radio-communications Consultative Committee (CCIR), and theInternational Standards Organization (ISO), and reference protocols for high-speedpacket communication are being st

45、andardized through ISO, CCITT, and the Institute of Electrical and Electronics Engineer (IEEE).Various factors such as these have made broadband communication realizable.5Therefore, the 1990s is the decade in which matured broadband communicationtechnologies will be used in conjunction with broadban

46、d standards to realize broadband communication networks. In the broadband communication network, the fiber opticnetwork will represent the physical medium for implementing broadband communication, while synchronous transmission will make possible the transmission of broadband service signals over th

47、e optical medium. Also, the B-ISDN will be essentialas the broadband telecommunication network established on the basisof optical medium and synchronous transmission and ATM is the communication means that enables the realization of the B-ISDN. The most important of the broadband services to be prov

48、idedthrough the B-ISDN are high-speed data communication services and videocommunication services.Image AcquisitionA TV camera is usually used to take instantaneous images and transform them into electrical signals, which will be further translatedinto binary numbers for the computer to handle. The

49、TV camera scans one line at a time. Each line is further divided into hundreds of pixels.The whole frame is divided into hundreds (for example, 625) of lines.The brightness of a pixel can be represented by a binary number with certain bits, for example, 8 bits. The value of the binary number varies

50、from 0 to 255, a range great enough to accommodate all possible contrast levels of images taken from real scene.These binary numbers are sorted in an RAM (it must have a great capacity) ready for processing by the computer.Image ProcessingImage processing is for improving the quality of the images o

51、btained. First, it is necessary to improve the signal-to-noise ratio. Here noise refers to any interference flaw or aberation that obscure the objects on the image. Second, it is possible to improve contrast, enhance sharpness of edges between images through various computational means.Image Analysi

52、sIt is for outlining all possible objects that are included in the scene. A computer program checks through the binary visual information in store for it and identifies specific feature and characteristics of those objects. Edges or boundaries are identifiablebecause of the different brightness leve

53、ls on either side of them.Usingcertain algorithms, the computer program can outline all possible boundaries of the objects in the scene. Image analysis also looks for textures and shadings between lines.Image ComprehensionImage Comprehension means understanding what is in a scene. Matching the prest

54、ored binary visual information with certain templates which represent specific objects in a binary form is technique borrowed from artificial intelligence, commonly referred to as "templeite matching"emplate matching? One by one,the templates are checkedagainst the binary information repre

55、senting the scene. Once a matchoccurs, an object is identified. The template matching process continues until all possible objects in the scene have been identified, otherwise it fails.通信系统一般的通信系统由下列部分组成：信源。它产生信息，可以是写成的或口头的文字，或是某种形式的数 据。发信机。发信机把信息转换为信号，信号形式要适合在通信信道中传输。通信信道。通信信道用于从发信机传送信号到接收机的媒质。信道可

56、以是无 线电链路或者直接的有线连接。接收机。接收机可以看作与发信机相反的东西。它把接收到的信号变回成信息，并把信息传给终端，这个终端可以是一台扬声器、电传打字机、或一个计算机数据库。所有通信信道的一个不好的特征是信号上都叠加了噪声。这种人们不希望存在的噪声会引起电话中声音失真，或者在电报报文或数据中出现的差错。频分复用频分复用是一种模拟技术，一个话音信号的频段在。3千赫，用单边带振幅 调制:SSB）可以把话音信号搬移到新的频段。例如，采用这种调制，搬移4路 相类似_的信号，使之均分520千赫频段。各路之间的间隙成为保护间隔，这些 间隔使得在 实际系统中允许有点频率误差及滤波不足等缺陷。一旦这个

57、新的基带信号：由4路组成的“群”：形成后，它就作为一个单一 的单 元在主干网中传送。可以构成一个分级的体系，即以若干路组成一个“群”，若干 “群”组成一个“超群”，再以若干“超群”组成一个“主群”或“极群”。通信设备把群或超群作为一些单一的单元来传送。对于无线电设备来说，并 不需知道含有多少路。只要有足够的带宽，无线电可以处理一个超群。群的大小 采取 折中考虑确定：因为处理每一路都需要独立的滤波器、调制器及振荡器 ，这比每个 群所需的这些设备要多得多，但是若有一个部件失效，则将失去与这个群 相关的全 部话路。时分复用采用脉冲编码调制时，有可能利用抽样值之间的时间来传送来自其它电路的信号，这种技

58、术称为时分复用,TDM)0要做到这一点，必须在通信链路的两端采 用同步 开关，使对来自各路的抽样值能一次轮流传送，于是好几个用户看起来都 像在同时 使用这条链路，虽然每个用户只有一些周期性的短时隙，抽样值之间的 原来的模拟 信号却可在接收机中重新构成。脉冲编码调制在模拟调制中，用信号来直接调制载波的振幅或频率。但在数字调制中则是 产 生一个脉冲流来代表原来的信号，然后用这个脉冲流来调制载波，或者直接在电缆 中传输。脉冲编码调制是常用的两种数字调制技术之一。所有的脉冲系统取决于按规定的时间间隔抽样的模拟信号的波形。对模拟话 音输入取样所产生的信号称为脉冲调制信号。在实际中它不是很有用，但可用作进步形成脉冲编码调制信号的一个中间阶段。数字调制的大多数优点来自发送脉冲只有两个电平，这种系统称为二进制系统。在PCK，把每个抽样值的大小变 换成一个二进制数。脉冲编码调制过程有三步：抽样、量化和编码。光纤通信广义地说，把信息从一点传送到另一点就称为通信。当信息跨越一段距离被传送时,就需要一个通信系统。在通信系统中，信息传送时通过把信息叠加在电 磁波 上或对电磁波进行调制来实现，电磁波起着载送信号的作用。这一经过调制的载波随后被传送到要求到达的目的地，在那里被接收，并且通过解调还原成原 始信息。 在运用电磁载波的领域，高新技术得到进一步的发展，比如射频、微波 以及毫米波 的频率都被用